1. Field of the Invention
Ceramic aerogels are among the most highly porous and lowest density materials. Their high porosity means that 95% or greater of the total bulk volume of a ceramic aerogel is occupied by empty space (or air), producing excellent thermal as well as sound insulating qualities. In addition, their high specific surface area (e.g. on the order of 600-1000 m2/g) should make them well suited for numerous applications, including as adsorbent beds for chemical separations, as catalyst supports, as platforms for solid state sensors, etc. Unfortunately, conventional ceramic aerogels are physically and hydrolytically very unstable and brittle. Their macro-structure can be completely destroyed by very minor mechanical loads or vibrations, or by exposure to moisture. In addition, over time these materials tend to produce fine particles (dusting) even under no load. Consequently, there has been little interest in ceramic aerogel monoliths for the above-mentioned as well as other applications, despite their excellent insulative properties, simply because they are not strong enough to withstand even minor or incidental mechanical stresses likely to be experienced in practical applications.
2. Description of Related Art
U.S. Patent Application Publication No. 2004/0132846, the contents of which are incorporated herein by reference, describes an improvement wherein a diisocyanate is reacted with the hydroxyl groups prevalent on the surfaces of secondary (φ 5-10 nm) particles of a silica aerogel to provide a carbamate (urethane) linkage. Additional diisocyanate monomers are further polymerized to produce a network of polyurea chains between the carbamate linkages of respective pairs of hydroxyl groups present on the secondary particles, resulting in a conformal polyurea/polyurethane coating over the silica backbone. The resulting structure was found to have only modestly greater density than the native silica gel (2-3 times greater), but more than two orders of magnitude greater mechanical strength, measured as the ultimate strength at break for comparably dimensioned monoliths.
More recent work has demonstrated that the production of such polymeric conformal coatings is not limited to diisocyanate linkages anchored from surface-native hydroxyl groups. Alternative polymeric architectures have also been shown to produce conformal coatings that dramatically improve the strength of ceramic aerogels. Specifically, non-native functional groups (e.g. amine or vinyl groups) can be incorporated into the surfaces of aerogel secondary particles and used as anchors for other polymeric cross-linking chemistries (such as epoxy and styrene). Methods and chemistries for such alternative polymeric cross-linking architectures are described herein as well as in co-pending, commonly-owned U.S. patent application Ser. No. 11/266,025 and the following publications the contents of all of which are incorporated herein by reference in their entirety:    1. Structure-Property Relationships in Porous 3D Nanostructures as a Function of Preparation Conditions: Isocyanate Cross-Linked Silica Aerogels. Meador, M. A. B.; Capadona, L. A; McCorkle, L.; Papadopoulos, D. S.; Leventis, N., Chem. Mater. 2007, 19, 2247-2260.    2. Flexible, low-density polymer crosslinked silica aerogels. Capadona, L. A., Meador, M. A. B., Alunni, A., Fabrizio, E. F., Vassilaras, P., and Leventis, N. Polymer, 2006, 47, 5754-5761;    3. Chemical, physical and mechanical characterization of isocyanate cross-linked amine modified silica aerogels. Katti, A.; Shimpi, N.; Roy, S.; Lu, H.; Fabrizio, E. F.; Dass, A.; Capadona, L. A.; Leventis, N. Chem. Mater. 2006, 18, 285-296.    4. Cross-linking amine modified silica aerogels with epoxies: mechanically strong lightweight porous materials. Meador, M. A. B., Fabrizio, E. F., Ilhan, F., Dass, A., Zhang, G., Vassilaras, P., Johnston, J. C., and Leventis, N., Chem. Mater., 2005, 17, 1085-1098.    5. Hydrophobic monolithic aerogels by nanocasting polystyrene on amine-modified silica. Ilhan, U. F.; Fabrizio, E. F. McCorkle, L.; Scheiman, D. A.; Dass, A.; Palczer, A.; Meador, M. A. B.; Johnston, J. C. and Leventis, N., J. Mater. Chem., 2006, 16 3046-3054.    6. Bridged Polysilsesquioxanes. Molecular-Engineered Hybrid Organic-Inorganic Materials. Loy, D. A.; Shea, K. J. Chem. Mater. 2001, 13, 3306-3319.    7. Bridged Polysilsesquioxanes. Highly Porous Hybrid Organic-Inorganic Materials. Loy, D. A.; Shea, K. J. Chem. Rev. 1995, 95, 1431-1442.    8. Sol-Gel Synthesis of Hybrid Organic-Inorganic Materials. Hexylene- and Phenylene-Bridged Polysiloxanes. Douglas A. Loy, Gregory M. Jamison, Brigitta M. Baugher, Sharon A. Myers, Roger A. Assink, and Kenneth J. Shea, Chem. Mater. 1996, 8, 656-663    9. U.S. Patent Application Publication No. 2006/021621    10. Aryl-Bridged Polysilsesquioxanes-New Microporous Materials. Kenneth J. Shea* and Douglas A. Loy, Chemistry of Materials 1989, 1, 572-574.    11. A Mechanistic Investigation of Gelation. The Sol-Gel Polymerization of Precursors to Bridged Polysilsesquioxanes. Kenneth J. Shea and Douglas A. Loy Acc. Chem. Res. 2001, 34, 707-716.    12. Tailored Porous Materials. Thomas J. Barton, Lucy M. Bull, Walter G. Klemperer, Douglas A. Loy, Brian McEnaney, Makoto Misono, Peter A. Monson, Guido Pez, George W. Scherer, James C. Vartuli, and Omar M. Yaghi. Chem. Mater. 1999, 11, 2633-2656.    13. U.S. Patent Application Publication No. 2007/0203341    14. U.S. Pat. No. 5,321,102    15. U.S. Pat. No. 6,284,908
The polymer cross-linked aerogels described and referred to above exhibit far greater strength than the corresponding native ceramic aerogels (as much as two orders of magnitude greater strength with only a two- to three-fold increase in density). However, despite their improved strength they still remain relatively inflexible and are subject to brittle failure. Consequently there are numerous applications that could benefit from the insulative and improved mechanical properties of ceramic aerogels as described in the aforementioned publications, but where additional flexibility is necessary or would be desirable. For example, space-suit insulation could benefit significantly from more flexible ceramic aerogels having the insulative properties described above.
Accordingly, it is desirable to produce ceramic oxide aerogels as above, but which exhibit a greater degree of flexibility.